Have you ever rejected a piece of chocolate because of its surface had turned an unappetising dusty white? You're not alone: that bloom is the reason many turn away from what is arguably the world's most beloved sweet indulgence, even though it's still safe to eat.

In an attempt to help reveal the process of the bloom, and thereby figure out a means of preventing it, scientists at German national research centre Deutsches Elektronen-Synchrotron (DESY), the Hamburg University of Technology (TUHH), and Nestlé (which funded the research) have turned a powerful X-ray to chocolate.

"Although fat blooming is perfectly harmless, it causes millions in damage to the food industry as a result of rejects and customer complaints," said study main author Svenja Reinke of TUHH. "Despite this well-known quality issue, comparatively little has been known until now about its root causes."

Implying DESY's PETRA III is just an ordinary X-ray machine would be doing it a huge disservice. It's hugely powerful -- the most powerful light source of its kind, and the most brilliant storage-ring-based X-ray radiation source in the world. The 2.3-kilometre particle accelerator is particularly useful for examining very small samples.

It is through the use of PETRA III that the researchers were able to study the underlying processes of chocolate bloom in real-time. The powerful machine was able to show the chocolate down to a scale of just a few nanometres, revealing that the bloom is caused by the migration of liquid fats (such as cocoa butter) to the surface of the chocolate, where it crystallises.

Fat bloom on chocolate as seen through a microscope.
Svenja Reinke/TUHH

"This can happen when liquid chocolate cools down in an uncontrolled manner and unstable crystals form, for example," Reinke said. "But even at room temperature, a quarter of the lipids [fat molecules] contained in chocolate are already in a liquid state."

The team first ground the chocolate to a fine powder, in order to investigate the behaviour of different mixes of the components of chocolate: cocoa, sugar, milk powder and cocoa butter. Powderising the chocolate speeds up the blooming processes.

Then a few drops of sunflower oil were placed on each of the chocolate samples, which sped up the fat migration process. The oil penetrated the chocolate very quickly, even to the smallest pores, altering the chocolate's internal structure. Over a few hours, the liquid fat dissolved the chocolate's lipid structures, making the entire chocolate structure softer, which in turn increases lipid migration.

This study will help the industry develop solutions for the prevention of chocolate bloom.

"One consequence might, for example, be to reduce the porosity of the chocolate during manufacture, so that the fat migrates more slowly," Reinke said. "Another approach is to limit the amount of fat that is present in a liquid form by storing the product in cool, but not too cold, conditions. 18 degrees Celsius [65 degrees Farenheit] is ideal."